Animals require light to align their physiological and behavioral activities to the light:dark cycle. In humans, disruption of this synchronization can lead to physiological and behavioral disorders that affect mood, sleep- wake cycles, cancer development and treatment. In addition to the classical photoreceptors, rods and cones, a small subset of retinal ganglion cells is intrinsically photosensitive in the mammalian retina. These ganglion sells express the photopigment Melanopsin (Opn4), which is responsible for mediating circadian and other ion-Image forming light responses in mice. Phylogenetic analysis and sequence similarity demonstrate that vertebrates evolved two Melanopsin groups: Opn4x, similar to the first Melanopsin discovered in Xenopus, and Opn4m, similar to the mammalian protein. Mammals have only one melanopsin gene (opn4m) and its expression is restricted to retinal ganglion cells (RGCs). I have identified five opn4-related genes in the zebrafish that show unique and non-overlapping patterns in the retina. In addition, all ofthe genes are expressed transiently in distinct regions ofthe developing zebrafish brain. The zebrafish opn4x2 gene is expressed in a pattern similar to the mammalian gene, but also is expressed in a small number of cells at the 3ase ofthe phototoreceptive organ, the pineal gland. The goal of this study is to identify and explore the unctions of opn4-expressing cells in the retina. 1 aim to 1. Determine the absorption spectra ofthe five zebrafish proteins and capability to respond to light, 2. Determine the expression pattern and identity ofthe 3pn4-expressing cells by in situ hybridization and immunofluorescence with known retinal markers of opn4x2 autative retinal ganglion cells, 3. Generate stable transgenic lines using Tol2 transposition to map the neural circuitry of opn4x2 and identify the brain targets and 4. Ablate the opn4x2-expressing cells selectively and Derform behavioral assays to assess the function of these cells. These studies will provide insights into the diverse roles of retinal opn4-expressing cells in influencing the circuitry and function of visual and non-visual ight responses. In addition these studies will further our understanding of the retinofugal connectivity of opn4-expressing cells in a diurnal, non-mammalian vertebrate.